1. Field of the Invention
This invention relates generally to crystal growing systems utilizing a melt and a crystal seed that is withdrawn therefrom. More particularly, this invention relates to a feeder for controlling the flow of pelletized solid charge material into the melt to replenish the melt simultaneously with continuing crystal growth.
2. Background Discussion
Several techniques are known in the art for growing crystals. The Czochralski process is the most widely used of these processes and is generally summarized below. A heated crucible is provided for holding a melted form of a charge material from which the crystal is to be grown. The melt is maintained at a temperature slightly above that at which the charge material crystallizes. A seed is placed at the end of a cable or rod that will enable the seed to be lowered into the melt material and then raised back out of the melt material. The seed can be either a sample of the desired crystal material or any other material that has the same crystalline structure and a higher melting temperature. When the seed is lowered into the melt material, it causes a local decrease in melt temperature, as is known to those skilled in the art, which results in a portion of the melt material crystallizing around and below the seed. Thereafter, the seed is slowly withdrawn from the melt. As the seed is withdrawn, the portion of the newly formed crystal that remains within the melt essentially acts as an extension of the seed and causes melt material to crystallize around and below it. This process continues as the crystal is withdrawn from the melt, resulting in crystal growth as the seed is continually raised.
As described above, crystal growth results from crystallization of the melt material. Therefore, as the crystal grows, the amount of melt material contained within the crucible is correspondingly decreased. Many prior art crystal growing systems were incapable of replenishing the melt simultaneously with continuing crystal growth. Consequently, the systems suffered from a number of disadvantages that are more fully described below.
In a system utilizing the Czochralski process, the crucible is located within a furnace that heats the crucible around its exterior surface resulting in thermal convection and a non-uniform temperature field. Therefore, the crucible is rotated in an attempt to provide even heat distribution throughout the melt. However, because the crucible is heated only around its exterior surface, changes in the melt level within the crucible result in changes in the thermo gradients of the melt. Any change in the thermo gradients during the growth of a crystal is undesirable because it changes the growing conditions for the crystal and results in a crystal that does not have uniform properties over its entire length. In an effort to maintain uniform growing conditions as the melt level decreased, a number of prior art systems correspondingly raised the crucible as the crystal was raised However, as the crucible was raised, the transfer coupling between the heating system and the crucible necessarily changed which resulted in changes in the thermo gradients of the melt. Additionally, this attempted solution added complexity and cost to the system by requiring that some apparatus be provided for raising the crucible
A further disadvantage resulting from the inability to replenish the melt is that system operation is delayed whenever the melt material is exhausted. When the melt material is exhausted, the furnace must be cooled down, cleaned, reloaded and then reheated to the appropriate temperature. As a result, these systems experience long cycle times with only a fraction of the cycle time being devoted to actually growing crystals.
In view of the foregoing, systems have been developed that allow the melt material to be replenished simultaneously with continuing crystal growth. An example of such a system is disclosed in Japanese Patent No. 49-10664 issued to Yamashita. In the Yamashita system, the addition rate of replenishing pellets into the melt is controlled by a vibratory system A feedback mechanism detects the level of the melt and sends a control signal to the vibratory system to specify the amount of charge material to be added to the crucible. The charge material is provided in the form of solid pellets The feeder system provides the pellets on a horizontal plate that is capable of undergoing vibration. When the melt level is sufficient so that no charge material need be added, a command is sent to the horizontal plate that stops it from vibrating and thereby ensures that no pellets will be displaced off of the plate and into the crucible Conversely, when the melt level is insufficient so that it needs to be replenished, a command is sent to the horizontal plate which causes it to be vibrated, thereby causing pellets located on the plate to be displaced off of the plate and into the crucible The number of pellets that are displaced off of the plate is related to the intensity with which the plate is vibrated.
The Yamashita system suffers from a problem relating to its inability to precisely control the amount of replenishment material fed into the system. In order to maintain constant growing conditions for the crystal, it is desirable to maintain the melt at a constant level Therefore, the addition rate of the replenishing material should be equal to the growth rate of the crystal The Yamashita system cannot precisely control the addition rate of the replenishing material because of the type of feeder system it utilizes. The charge material is provided in the form of pellets that necessarily have some variation in size. Different size pellets respond differently to the vibrations that are imparted thereto by the vibrating plate. Therefore, the vibratory system tends to sort the pellets by size. As a result, for any given duration and intensity level of vibration, the amount of charge material that will be sourced into the melt is not fixed. Rather, the addition rate is dependent upon the size of the pellets that happen to be on the vibratory plate at any given time. Consequently, the amount of replenishment material added to the melt cannot be controlled with specificity which results in the system being unable to maintain the melt at a constant level with the required degree of precision.
Another type of prior art feeder system is disclosed by Lane in European Patent Application number 0170856. Lane discloses the use of a storage hopper for storing particles of solid charge material The hopper is provided with an opening at the bottom thereof which allows the particles to flow out of the hopper. A conveyor belt is provided below the opening and collects the particles that flow out of the hopper. In this manner, piles of the particles are formed between the conveyor and the opening. The conveyor belt has rollers positioned at each end that advance the belt which thereby advances the particles piled thereon. As the conveyor advances, particles of charge material are displaced off the edge of the belt and into the crucible to replenish the melt.
The Lane system improves upon some of the disadvantages of the Yamashita system because the feed rate of the Lane system varies less in response to changes in individual particle sizes. However, it is believed that the Lane design has a number of potential problems. First, as is more fully described below, an important consideration in designing crystal growing systems is minimizing contamination of the charge material. The Lane belt must be made from a flexible material in order to to operate properly and it is difficult to find a flexible material that is non contaminating with respect to many common charge materials. Second, the system utilizes a number of moving parts contained within the enclosed portion of the system. For reasons that are more fully described below, it is desirable to enclose the portions of the crystal growing system that come into contact with the charge material within a vacuum chamber. The charge materials used to grow crystals, such as silicon, are often quite abrasive. As a result, abrasive dust particles of the charge material are plentiful inside the enclosure. These abrasive dust particles can build up between moving parts of the system and cause grinding therebetween. Since the moving parts are generally formed from a contaminating material, the grinding of the moving parts can cause particles of that material to become chipped off, which eventually results in the chipped particles entering the melt and causing the contamination thereof. Additionally, the grinding of the moving parts can result in a need to replace the moving parts frequently. Therefore, it is desirable to reduce the number of moving parts that are contained within the enclosure in order to reduce costs associated with maintaining the system. The Lane system has at least two moving parts, i.e. the two rollers for the conveyor belt, located within the enclosure. Third, the Lane belt necessarily has some sag associated with its center portion when a pile of particles is placed thereon. As a result, the distance between the belt and the hopper can vary depending upon the amount of particles piled on the belt. For reasons that are more fully explained below, this variation in pile size can undesirably vary the feed rate of the system.
Accordingly, it is an object of the present invention to provide, for a crystal growing system, an improved feeder system that provides precise control over the addition rate of pelletized charge material for replenishing the melt simultaneously with continuous crystal growth, and further improves upon the problems associated with the prior art systems.